Various anticholinergic compounds have been previously described but are not optimal.
Muscarinic receptor antagonists are frequently used therapeutic agents that inhibit the effects of acetylcholine by blocking its binding to muscarinic cholinergic receptors at neuroeffector sites on smooth muscle, cardiac muscle, and gland cells as well as in peripherial ganglia and in the central nervous system (CNS). However, their side effects, which can include dry mouth, photophobia, blurred vision, urinary hesitancy and retention, decreased sweating, drowsiness, dizziness, restlessness, irritability, disorientation, hallucinations, tachycardia and cardiac arrhythmias, nausea, constipation, and severe allergic reactions, often limit their clinical use, and even topical anticholinergics can cause the same unwanted side effects. Glycopyrrolate and triotropium are among the quaternary ammonium anticholinergics, which have reduced CNS-related side effects as they cannot cross the blood-brain barrier; however, because glycopyrrolate (or, presumably, tiotropium) is eliminated mainly as unchanged drug or active metabolite in the urine, its administration is problematic in young or elderly patients and especially in uraemic patients. To increase the therapeutic index of anticholinergics, the soft drug approach has been applied in a number of different designs starting from various lead compounds over the past 20 years, but there is a need for yet other new soft anticholinergics. These novel muscarinic antagonists, just as all other soft drugs, are designed to elicit their intended pharmacological effect at the site of application, but to be quickly metabolized into their designed-in, inactive metabolite upon entering the systemic circulation and rapidly eliminated from the body, resulting in reduced systemic side effects and increased therapeutic index.